Since carbon fibers have a high level of specific strength and a high level of specific tensile modulus, as well as a feature of having a low specific gravity because of including mainly carbon elements, carbon fibers as reinforcing fibers for a composite material are being widely developed to applications for general industries such as automobiles, civil engineering and construction, pressure vessels and windmill blades, as well as conventional applications for sports and applications for aircraft or space, and there has been a high demand to achieve both further technical advancement in performance and cost reduction. In particular, there has been a need for a carbon fiber having a smaller specific gravity, by which the weight is reduced as much as possible, as well as having enhanced mechanical properties, mainly tensile modulus of resin-impregnated strand (hereinafter, referred to simply as strand).
The most widely utilized polyacrylonitrile-based carbon fiber is industrially produced through a stabilization process of converting a polyacrylonitrile-based precursor fiber into a stabilized fiber under an oxidizing atmosphere at 200 to 300° C. and a carbonization process of carbonizing the fiber under an inert atmosphere at 300 to 3000° C. For the purpose of decreasing the specific gravity of the carbon fiber, there is a method of widening the interplanar spacing of the graphite crystallite and a method of lowering the specific gravity of a structure of a portion other than the crystallite.
In Japanese Patent Laid-open Publication No. 3-241014 and Japanese Patent Laid-open Publication No. 2006-283226, there has been proposed a technique of allowing carbon fiber to have a single hollow portion as the core portion thereof to control the apparent specific gravity at a low level.
Moreover, usually, the carbon fiber allows a large number of voids to be contained in the inside of the fiber during the production process. The volume of voids and the size of the voids are important factors in determining the mechanical properties of the carbon fiber and the carbon fiber strand such as the tensile strength thereof and the tensile modulus thereof. As such, from an aspect of attaining the enhancement in physical properties of the carbon fiber, it is important to control the volume of voids and the size of the voids contained in the inside of the carbon fiber.
In Japanese Patent Laid-open Publication No. 2010-229573, there has been proposed a technique of controlling tension applied to a fiber at a temperature within a specific temperature range in a carbonization process to control the volume of voids contained in the inside of the carbon fiber and attaining enhancement of the tensile strength of resin-impregnated strands and the tensile modulus of resin-impregnated strands.
However, in the techniques proposed in Japanese Patent Laid-open Publication No. 3-241014 and Japanese Patent Laid-open Publication No. 2006-283226, while the apparent specific gravity of the carbon fiber is controlled at a low level, the tensile strength of resin-impregnated strands and the tensile modulus of resin-impregnated strands thereof are still insufficient since the carbon fiber having a single hollow portion is a flaw in itself.
Moreover, the technique proposed in Japanese Patent Laid-open Publication No. 2010-229573 seeks to extremely reduce the void amount in the inside of the carbon fiber while maintaining a high level of tensile strength of resin-impregnated strands and a high level of tensile modulus of resin-impregnated strands, and the carbon fiber has not been sufficiently reduced in weight.
Accordingly, there have not been any carbon fibers having high mechanical properties while being light in weight and a production method therefore.
It could therefore be helpful to provide a carbon fiber capable of achieving a high level of elongation and a high level of tensile modulus of resin-impregnated strands while being light in weight.